It all comes down to how they define their own identities.

In one study of four private religious schools, students and faculty were overwhelmingly creationist, but in the Muslim schools, creationism mattered less.

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For some Christian creationists, the scientific theory of evolution is an existential threat to their faith—one they feel they must publicly oppose. “There is a war in society,” writes Ken Ham, the founder of Kentucky’s Creation Museum, in an excerpt of his book. “The essence of the conflict lies firmly at the foundational level—creation versus evolution.”

Many creationists take this “conflict” extremely seriously. In Slate, I’ve written about public school biology teachers in Bossier, Louisiana, who are teaching their students about the “Genesis point of view” and a massive charter system in Texas whose textbooks called evolution “dogma” and said the fossil record was “sketchy.” They do this despite the fact that teaching creationism in public schools is illegal and could land these school districts with huge lawsuits. In 2005, Pennsylvania’s Dover Area School District paid more than $1 million in legal fees and damages after losing in court because it was teaching intelligent design creationism.

But Christian creationists can’t stop pushing creationism. In 2014, Louisiana’s Sabine Parish School District settled its own lawsuit after a teacher repeatedly bullied and humiliated a –sixth-grade Buddhist student for not believing in creationism. As part of the settlement in Sabine Parish, the bullied student transferred to a new school away from his harassers. But even at his new school, “The kids were touching him with their crosses to see if he would melt,” said his father, Scott Lane.

What is it that drives Christian creationists to keep publicly contesting evolution and to react so cruelly to people who don’t believe? Recent research suggests that this fight is about identity.

“Our identities are formed by what we do and who we distinguish ourselves from,” said Jeffrey Guhin, a sociologist and professor at UCLA who has studied creationists. Guhin thinks that a determining factor in whether a creationist will actively promote his belief stems from how emotionally connected his belief in creationism is to his identity.

Vocal creationism is part of how some Christian creationists reinforce their sense of self and create a social hierarchy that allows them to make sense of the world, he posits.

In my years debating and reporting on creationists, I have found they regularly ask: “Why have we [humans] stopped evolving?” This is an attempt to engage with what creationists believe are science’s terms about evolution, but the question itself reveals the underlying social need to vocally oppose evolution. The reality is that humans haven’t stopped evolving (our life spans are just too short to see human evolution), but to a creationist without a background in evolutionary science, this question makes sense.

Evolving or not, humanity must be special or on top of the mortal dog pile, so to speak. It’s a throwback to an old Biblical classification scheme, the Great Chain of Being, which organized everything in heaven and on Earth. God and the angels were on top, but below them in the chain were humans, God’s ultimate creation, who were above every other Earthly thing, from dogs to rocks. “And let [men] have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth,” says Genesis 1:26 of the King James Bible.

If humans were perfectly created to be in charge, evolution can’t be true, and all that’s left for scientists to do is define each organism’s place in the world. (Unsurprisingly, creationist textbooks are big on classification.) That’s exactly what creationism does—promoting creationism is the political equivalent of shouting, “I’m significant!” at the night sky. When considered this way, you can see why people would be compelled to continue the fight.

But not all creationists care about spreading the gospel of creationism. “Ideas are not issues by default,” says Guhin. He’s the author of a new study published this June in the journal Sociological Theory that expands our understanding of how denialism and identity interact.

What’s interesting is how this breaks down across religions.

Guhin spent a year and a half studying the differences between two Sunni Muslim and two conservative Protestant private schools in New York City. The students and faculty at all four schools were overwhelmingly creationist, but in the Muslim schools, creationism mattered less. “Evolution came up regularly during Bible and science classes at Good Tree and Apostles [the two Christian schools],” wrote Guhin. “It only came up twice without me bringing it up at Al Haqq and never at Al Amal [the two Muslim schools].” In the Muslim schools, Guhin was the person who broached creationism. “Even in biology classes at the Muslim schools, the religious significance of evolution was usually glossed over and only cursorily acknowledged, if at all,” he wrote.

Guhin asked a teacher at Al Amal why he didn’t “talk that much in class about evolution’s religious implications.” The teacher’s answer was, “I just don’t know about it; it’s not my subject area.” Another teacher called it a “minor issue” and was unwilling to even call it “haram” (prohibited in Islam), because it was such a minor topic.

“It is hard to imagine a Conservative Protestant at either of [the Protestant] schools—particularly a biology teacher—claiming not to know what their religion teaches about evolution,” wrote Guhin in his paper.

He theorized that the reason creationism is contested so publicly by American Protestants, but not really addressed by American Muslims, is because of how each community defines its boundaries with the broader public.

Boundaries are how societies define their differences with outside groups. Their inverse are practices, which are the similarities between members of the group. “For evangelicals the key boundary and key practice was reading the Bible literally,” said Guhin, and creationism is a literal interpretation of the book of Genesis. This is why opposition to evolution is important to Christian creationists. It’s how they police who is in and who is out.

In the Muslim schools, there were different important practices and boundaries, including prayer and gender roles. “The key practice for the Muslims was prayer, what they do, Salah, five times a day,” Guhin said. “The key demarcation from the outside world was gender performance, like how we interact with people of the opposite sex.”

Evolution is “neutral” when it comes to prayer and gender performance, Guhin said, meaning it’s not part of forming a group identity for American Muslims. Some Muslims are Quranic literalists, but most students that Guhin interacted with viewed prayer as more important than emphasizing the Quran in a way that would make creationism central. One teacher at Al Haqq even lamented to Guhin that many of the school’s Qurans have never been opened. “Part of the explanation for why those Qur’ans stay on the shelf is the centrality of prayer,” he wrote.

There’s a larger theological distinction that’s helped create these different boundaries. Conservative Protestants believe in being saved by grace, which is the theological concept that faith in God alone is enough to land you in heaven (and faith is also the only factor). Emphasizing creationism is emphasizing faith. Sunni Muslims believe they’re saved by their actions (along with their beliefs), and prayer is “perhaps the most important” action, wrote Guhin. That’s why prayer and wearing clothes such as the hijab are more important identity markers for Muslims than evolution.

But for Christian creationists, they fight this battle over evolution because they have to. For them, they’re not just trying to save souls—they’re fighting a battle for their own souls.